Television transmitting tube



Jan. 13, 1942. H. A. IAMs I 2,269,588

TELEVISION TRANSMITTING TUBE v Filed Deo. 21 1940 INVENTOR. HARLEY .4'. JAA/l5 ATTORNEY.

Patented Jan. 13, 1942 TELEVISION TRANSMITTING TUBE Harley A. Iams, Summit, N. J., assignor to Radio Corporation of America, a corporation of Dela- Ware Application December 21, 1940, Serial No. 371,108

(Cl. Z50-150) 9 Claims.

for compensating for second order distortions which are introduced into the television signals produced by reason of the scanning of an electrostatic charge storage type of mosaic electrode in an electronic image transmitting tube which is subjected both to the light of an image from which the electro-optical image is to be produced, and to the action of a high velocity electron scanning beam used to discharge the mosaic electrode.

In a tube and system of the type to which the present invention is directed, it frequently happens that while it is possible and practical to generate and transmit television image signals in which there is substantially a complete absence of irst order distortion, known systems are not entirely free from second order distortions. One objectionable second order distortion eiect is that which is known in the art as black spot or dark spot distortion. By dark spot distortion is meant the production on the viewing plane at the points of reception of an image in which there is dark shading frequently distributed at random points or small areas about the viewing area of the receiver observation screen.

Such distortion tends to cause both an unduly dark image particularly adjacent the borders and at many places on the receiver viewing screen and also tends to make the highlights of the image at areas, where dark spot is observable,

vless bright than should be the case.

In such television transmitting apparatus use vis made of a cathode ray tube wherein there is provided means for developing and moving in desired co-ordinates an electronic scanning put circuit receives the signal energy in accordance with a varying voltage drop across a resistor connected in series with the signal plate. Supported directly upon or adjacent the signal plate is an insulating member or di-electric sheet, such as a sheet of mica, which supports photosensitive material capable of releasing primary photo-electrons in accordance with the light of the optical image impinging thereupon. The photosensitive material is formed as a series of minute-size particles, each electrically insulated one from the other, as well known in the art.

The optical image of which an electro-optical replica is to be produced is projected directly upon this photosensitive portion of the mosaic electrode and the release of the photo-electrons causes electrostatic charges to build up between the photo-electric particles andthe signal plate. The stored electrostatic charges are neutralized or released when the cathode ray beam traverses and scans the photosensitive particles of the mosaic electrode. The charge accumulated and then released represents the light value of related elemental areas of the optical image and causes the production of signal modulations which constitute the video signals for transmission. However, during the time when the scanning electron beam is traversing the mosaic electrode, there develops frequently a spurious or undesired signal which causes the above-mentioned phenomena of darkened areas of the image reproducing screen of the receiver. Such dark spot signal is believed to result from the various electrical iield distributions occurring across the surface of the mosaic electrode, which cause the escape of secondary electrons released by the beam from the mosaic to be non-uniform.

It has been found in practice that the dark spot phenomena usually result from the character oi the photosensitive portion of the mosaic electrode. While the dark spot characteristics are not entirely uniform from one tube to the next, there is nevertheless a considerable degree of relationship between the characteristics of different scanning tubes, and the dark spot signal resulting from tubes of similar physical size and shape is usually closely related in position of occurrence on the viewing screens at the receivers or monitors. While it is possible to provide a substantial degree of compensation for the dark spot phenomena through the use of two separate scanning tubes operating simultaneously and producing identical dark spot signals which are combined in phase opposition, the use of two such tubes introduces additional space and circuit requirements which are disadvantageous from the standpoint of operation.

Accordingly, it is an object of my invention to provide a simplified apparatus and means for compensating for objectionable dark spot phenomena in a reproduced television image. It is also an object to provide a single tube incorporating means for dark spot compensation which will not require the continuous operation of additional compensating tubes. It is a further object to provide a structure including within a single envelope means for dark spot compensation, and a still further object to provide a structure which may be processed for spurious signal effects during actual operation to reduce generation of dark spot signals. These and other objects, features and advantages of my invention will be apparent when read in connection with the following description andthe accompanying drawing in which:

Figure 1 is a cross-sectional view of a television transmittingr device made in accordance with lmy invention,

Figure 2 is a sectionalized view of a target electrode of the mosaic type made in accordance with my invention,

Figure 3 is a tube of a type which may be processed during use to control the generation of dark spot signals,

Figure 4 is a cross-sectional View of the tube shown in Figure 3 taken along the lines 4 4, and

Figure 5 is a sectionalized view of another form of target suitable for use in the tubes of Figures l and 3.

As indicated above, a considerable degree of lrelationship exists with respect to similarity of in tubes of similar design under construction. j-

which is scanned by an electron beam and on Which is focused the optical image of which a replica is to be transmitted, and on the opposite side of the target means which, when scanned by another electron beam, produce a signal directly opposite to that of the dark spot signal, thereby neutralizing the dark spot second order effects.

With particular reference to a tube made in `accordance with one teaching of my invention as shown in Figure 1, the tube comprises a highly evacuated envelope or bulb l having a cylindrical portion including a Amosaic electrode 2 and two tubular arms or neck sections, each including a conventional-type electron gun 3 and 3a. The

`mosaic electrode 2, thereinafter more fully described, is so mounted in the cylindrical portion of the envelope that it may receive light from an object, such as represented by the arrow 4, projected through the window of the tube and upon the mosaic electrode such as by the lens system 5. The electron guns 3 and 3a in the neck sections of the envelope each comprise a cathode neck portions of the envelope.

I-'la, control electrode -Ba connected to the usual biasing battery and a first anode 9-9a maintained positive with respect to the respective cathode l--la by a potential source or battery Ill. The electron stream leaving the rst anode 9 is accelerated and concentrated into an electron scanning beam focused on one side of the mosaic electrode 2, which will hereinafter be referred to as the front surface, by a second or beam focusing anode Il which is preferably grounded at thebattery IU. Similarly,the electron stream leaving the first anode 9a of the electron gun 3a is accelerated and concentrated into an electron scanning beam focused on the other side, which will hereinafter be referred to as the rear surface of the mosaic electrode 2, by a second or beam focusing anode Ila. Each of the second anodes II-Ila may be energized from the potential source or battery lil and need not be operated at the same potential depending upon lthe desired velocity of each of the electron beams. It is desirable to operate the second'anode Ha slightly positive with respect to the potential applied to the anode Il for the more efiicient collection of secondary electrons generated by the electron beam from the gun 3a impinging on the rear surface of the mosaic electrode. Such operationprevents substantial redistribution of secondary electrons over the rear surface areas of the mosaic electrode. Obviously, two potential sources, one for each of the second anodes and/or the associated electron gun structures may be utilized if desired. To scan the beam from the electron gun 3 over the front surface of the mosaic electrode and the other electron beam over the rear surface of the mosaic electrode I provide two sets of deflection coils, one of which may surround or partially surround the respective More particularly, I provide horizontal deflection coils I2 and vertical deflection coils I3 about the neck section including the electron gun 3 and similar deection coils l2a-I3a about the neck section including the electron gun 3a. The horizontal defiection coils about each neck section are operated synchronously, and similarly the vertical v'deflection coils about each neck section are operated synchronously so the electron beam from leach electron gun impinges upon the target over directly opposite elemental areas of the two sides of the target. Thus, as the electron beamlfrom the gun 3`scans the target over elemental areas on one side, the electron beam from the :gun 3a scans similar areas on the opposite side of the target.

Since the electron beam from the gun 3 is highly accelerated by the potential applied to the second anode Il, the beam impinges on the front surface of the target with a velocity sufricient to liberate secondary electrons, the principal portion of which is collectedby the second anode Il, the remaining secondary electrons being redistributed over the front surface of the target, such redistribution being non-uniform probably due to a non-uniform electrostatic field distribution opposite the front of the target, thereby producing the distortional dark spot effects.

In accordance with my invention I so construct the target electrode that elemental areas on the rear surface of the target have a secondary electron emitting characteristic whichy is directly opposite to the characteristic of the elemental areas istic of the elemental front areas are conducive to the generation of spurious signals such as dark spot signals.

Methods of processing and constructional details as to the principles underlying my invention will be understood by referring to Figures 1 and 2 and the following description of the particular mosaic-type target electrode shown therein. The mosaic electrode 2 includes an insulating foundation 20, such as a sheet of mica, having on one side thereof, such as the front surface facing the electron gun 3, a discontinuous photosensitive structure of the mosaic type such as a great plurality of mutually separated and insulated photosensitive particles 2l. I provide the rear surface of the mosaic electrode 2 with a conductive coating to serve as a signal plate 22 which is in capacitive relation with the particles 2| and is connected to an output circuit including an amplifying device 23 and to ground through a load impedance 24. In accordance with my invention the exposed rear side of the mosaic electrode 2 which may be the exposed surface of the signal plate 22 is so constructed and treated that the secondary electron emission therefrom is of such magnitude as to compensate for the variations in secondary electron emission causing dark spot effects on the opposite side or front surface of the mosaic such as from the photosensitive particles 2|. As indicated above, similarly processed tubes of similar physical size and shape usually have closely related dark spot phenomena both with respect to the intensity of the dark spot signal and with respect to the position or spacial location over which the dark spot signal occurs. From the knowledge obtained from tubes having similar-shaped envelopes and similarly positioned electrodes the occurrence and intensity of dark spot distortional effects may be quite accurately predetermined, and thus in accordance with my invention, I so treat the rear exposed surface of the mosaic electrode to provide dark spot compensating signals which are simultaneously generated with those signals causing dark spot and simultaneously applied to the same electrode on which the signals otherwise causing dark spot distortion are applied.

Referring to Figure 2 which shows a mosaic electrode having a rear surface treated to obviate dark spot difficulties, the exterior surface of the signal plate 22 may be treated with material 25 to vary the normal secondary emission characteristics of the signal plate. Thus the edge portions frequently appear abnormally bright in the reproduced picture replica and for this condition a darker image area appears on the central portion of the picture area which indicates an excess loss of secondary electrons over this darker image area. Consequently, to neutralize such dark spot signals the central portion of the signal plate should be made to liberate fewer secondary electrons than the areas over the edge portions of the target. More particularly as shown in Figure 2 the central areas may be coated with the material 25 such as carbon which has lower secondary electron emitting properties than the material of the signal plate 22 which may be a lm of platinum or other metal having known secondary emitting properties, The density, or amount per unit area, of the material 25 such asA carbon is greater over the central portion of the signal plate and decreases in density progressively toward the edges for this condition of spurious signal. A very convenient method of application of carbon to the signal plate is by printing. If this teaching is to be followed, a

photograph is made of the luminescent face of a cathode ray tube scanned by an electron beam having an impressed signal representative of the dark spot produced by a previously constructed tube having a structure similar, in so far as the structure to the front of the mosaic electrode is concerned, to the tube in which the mosaic electrode being processed is to be used. From the photograph a half-tone cut may be made and used for printing the rear exposed surface of the signal plate 22 using a material such as carbon ink for the printing vehicle.

Following the preparation of the electrode shown in Figure 2 the electrode may be sealed in the envelope or bulb l and photosensitized in the conventional manner such as by supporting the mosaic electrode 2 by a baille 26 which extends to the bulb wall. The electrons liberated from the front surface of the mosaic electrode 2 are collected by the second anode Il and the secondary electrons from the rear surface by the second anode Ila. It is undesirable that electrons liberated from one surface reach the other surface and the baffle 26 surrounding the mosaic electrode 2 and extending to the wall of the cylindrical portion of the envelope, as shown in Figures 1 and 3, prevents such electron flow. The baffle 26 may be a sheet of mica riveted or otherwise aixed to the target, leaving the front and rear surfaces of the target exposed to the scanning beam.

While I have described one method of constructing a mosaic electrode in accordance with my invention, the exposed surface of the signal plate need not be treated by a printing process. Thus if the rear surface is roughened over those areas where suppression of secondary emission is desired, a similar effect will be produced. Likewise, if the metal comprising the signal plate is of oxidizable material such as nickel, areas of the signal plate may be oxidized to various degrees to produce varying secondary electron emitting characteristics over the areas. It is therefore obvious that my invention broadly applies to providing a structure, one side of which is susceptible to spurious signal generation, the opposite side of which is so constructed so as to suppress such spurious signal generation.

While I have disclosed above various methods of manufacture wherein the tube under construction is manufactured in accordance with the knowledge of dark spot signal generation from previously constructed tubes, it is nevertheless possible in accordance with a further teaching of my invention to manufacture a tube without such prior knowledge. In accordance with this further teaching and with reference to Figures 3 and 4, I provide means within the envelope or bulb l to provide a non-uniform secondary electron emitting surface on the rear side of the mosaic electrode following construction of the tube and during the operation thereof. In accordance with this teaching I provide means to evaporate and condense material on the rear surface of the mosaic electrode, said material having different secondary emitting properties than the material comprising the rear portion of the electrode such as the signal plate 22. Thus I provide a plurality of evaporators 30 from which material may be vaporized and deposited by condensation such as upon the rear surface of the signal plate 22. Each of these evaporators may be similar to the sectionalized evaporator shown in Figure 3 and comprises a shield member 3l surrounding a filament 32 containing a pellet 33 of material to be vaporized and condensed on the signal plate 22. The evaporators 30 are so arranged lthat material vaporized therefrom is .directed toward various areas of the exposed surface of the signal plate 22. Each of theiilaments 32 has one of its leads sealed through the envelope wall, the other leads of each filament being connected together and to a common lead sealed in the envelope wall.

The tube of Figure 3 is constructed as well known in the art of manufacturing such tubes but includes two electron gun structures as shown in Figure l. Following completion, the tube is operated with each beam scanning the respective side of the target and the generated dark spot signals observed on the iiuorescent screen `of a cathode ray tube. While observing the dark spot signals, the laments 32 may be energized separately to vaporize and deposit material fromone or more of the pellets 33 on the rear surface of the signal plate 22 over the area where compensation for dark spot is desired. I have found `that materials when deposited by a vaporizing process have secondary electron characteristics which are different from similar materials applied by other processes. Thus if the material forming the signal plate 22 of the mosaic electrode is deposited by painting a metal or metal salt bearing liquid vehicle on the rear surface of the mica sheet followed by baking to form a metal deposit, as is the usual practice of applying such electrodes, a similar metal vaporized from the filaments 32 will have different secondary emitting properties. Either the areas of the signal plate 22 opposite the areas of the mosaic causing dark spot may be coated by the vaporizing material or the remaining areas may be coated, Adepending upon the properties of the deposited material as to whether the material when deposited decreases or increases the secondary electron emission with respect to the secondary emitting properties of the signal plate prior to coating. Various metals such as silver, nickel and palladium may be used for the pellet 33. In any event it is to be understood that by this latter teaching of my invention I am able to produce a tube. 1

wherein dark spot compensation may be obtained by observing the tube during operation and then so treating the tube as to substantially eliminate dark spot signals. This teaching is particularly advantageous in the construction of television,

transmitting tubes for specific uses such as for studio pickup, photographic lm pickup or outside pickup where the dark spot phenomenon is inuenced by the average amount of illumination used for the particular pickup. Thus a tube.

when being processed in accordance with my invention may be operated by projecting an average scene representative of scenes with which the tube is to be used, noting the dark spot signals thus produced under these average condi.

tions and depositing the material on the rear surface of the signal plate in accordance with such observation. Y

While I have described above an electrode wherein the signal plate 22 is coated with material such that the secondary emission'varies from area to area, the coating of material 25 may be deposited on or comprise a conductor separate from but co-extensive with the area occupied by the photosensitive particles. Thus as shown in Figure 5, wherein parts previously described are similarly referenced, the foundation may comprise two thin sheets of mica 2|] and 35 back to back with an intermediate signal plate 22, the exposed surface of the sheet 20 bearing the separated particles 2i and the exposedsurface of the other sheet 35 bearing a metal lm 36 treated to have a non-uniform secondary emitting surface 25 which may be either a variable secondary emitting nlm or may be an oxidized surface of the film 36. The film 36 may be in capacitive relation with the signal plate 22 or may be connected directly thereto so that the effect of nonuniform emission of secondary electrons will be imparted directly to the signal plate.

It will be noted that in both modications of my invention the surface or element exhibiting non-uniform secondary electron emission over exposed areas thereof is a uni-potential surface or element. My invention cannot be practiced to my knowledge by scanning an insulated surface or element of insulation because elemental areas would acquire charges which would vary the desired secondary electron emission. In addition, an insulated surface would be subject to considerable redistribution of secondary electrons, since it would be difficult, if not impossible, to maintain a definite electrostatic collecting field adjacent the rear surface of the mosaic electrode, such as the field produced between the rear exposed conductive coating 22 or 36 and the second anode Ila, when this anode is maintained positive with respect to the coating.

It will be appreciated that my invention possesses advantages over systems previously proposed utilizing a plurality of tubes and intricate circuit arrangements where the outputs of two or more tubes generating identical dark spot signals are combined in phase opposition for the purpose of dark spot compensation. Therefore, while I have described the preferred embodiments of my invention of which I am now aware and have indicated various specic applications for which my new tube may be employed, it will be apparent that my invention is by no means limited to the exact forms illustrated but that many variations may be made in the particular structure and methods of application used and the purpose for which it is employed without departing from the scope of my invention as set forth in the appended claims.

I claim:

1. A television transmitting tube comprising in an evacuated envelope a target electrode foundation, a first electron gun exposed to one side of said foundation to generate an electron beam, a light sensitive structure which inherently liberates secondary electrons when scanned by said beam on the side of said foundation exposed to said electron gun, a signal plate in capacitive relation with said photosensitive particles, a second electron gun positioned to direct an electron beam toward the opposite side of said foundation, a film of material adjacent the said opposite side cf said foundation and conductively associated with said signal plate, said lm of material having different secondary electron emitting properties over laterally spaced elemental areas thereof and means to scan said electron beams over directly opposite areas on opposite sides of said foundation to neutralize spurious signals generated by one of said electron beams.

2. A television transmitting tube comprising in an evacuated envelope a target electrode including a mosaic of mutually separated particles adapted to liberate secondary electrons when scanned by an electron beam, a signal plate in capacitive relation with said mosaic, a coating over areas of said signal plate having non-uniform secondary electron emitting properties over the exposed surface thereof, means to generate and scan an electron beam over said mosaic, thereby liberating secondary electrons which are distributed non-uniformly over the said mosaic, means to generate and scan a second electron beam over said coating to liberate secondary electrons in accordance with the nonuniform distribution of secondary electrons over said mosaic to neutralize the effect of said nonuniform distribution.

3. A television transmitting tube comprising an evacuated envelope, a target in said envelope dividing said envelope into two portions to prevent electrons flowing from one portion to the other, a mosaic of mutually insulated particles which when scanned by an electron beam produces picture signals mixed with spurious signals on one side of said target exposed to the interior of one envelope portion, a conducting plate havtaneously produce and neutralizespurious signals.

4.. A television transmitting tube as claimed in claim 3 including means in each envelope portion to collect secondary electrons independently of the electrons collected in each envelope portion.

5. A television transmitting tube comprising an evacuated envelope, a target in said envelope, a mosaic of mutually insulated particles on one side of said target, an electrically conductive coating on the opposite side of said target, means to generate and scan an electron beam over said mosaic to develop picture signals having spurious signal components caused by non-uniform distribution of secondary electrons over said mosaic, means over at least a portion of the exposed surface of said conductive coating to change the secondary emitting properties of said conductive coating in proportion to the spurious signals produced from areas of said mosaic and means to generate and scan an electron beam over said last-mentioned means to neutralize said spurious signal components.

6. A television transmitting tube comprising an evacuated envelope, two oppositely disposed electron guns to develop electron beams, a target intermediate said electron guns, a light sensitive structure on one side of said target facing one electron gun and an electrically conductive coating on the opposite side of said target and facing the other of said electron guns, the said light sensitive structure having the property of nonuniformly collecting secondary electrons over the area thereof, thereby producing spurious signals mixed with picture signals when scanned by an electron beam, and said conductive coating having higher secondary emitting properties over areas directly opposite areas of said structure c01- lecting a greater number of secondary electrons, and means to scan said electron beams over the sides of said target facing the respective electron guns whereby spurious signals generated by one electron beam are neutralized by non-uniform generation of secondary electrons during scansion by the other electron beam.

7. A television transmitting tube comprising an evacuated envelope, two oppositely disposed electron guns to develop electron beams, a target intermediate the said electron guns, a light sensitive structure on one side of said target in the path of the beam from one of said electron guns, an electrically conductive film having predetermined secondary electron emitting properties on the other side of said target in the path of the beam from the other of said guns, means to scan said electron beams over the respective sides of said target to generate picture signals mixed with spurious signals, means to vaporize material and form a coating over areas of the said conductive lm during operation of said tube and a coating desposited from said last-mentioned means on said conductive film to neutralize said spurious signals.

8. In combination with a television transmitting tube, a target having a light sensitive structure adapted to be scanned by an electron beam, a conductive coating on one side of said target, the exposed area of said conductive coating being non-uniformly secondary electron emissive from area to area thereof when scanned by another electron beam, and means to collect secondary electrons from said conductive coating without substantial redistribution of secondary electrons over the areas of said conductive coating.

9. In combination with a television transmitting tube, a target electrode having a light sensitive structure on one side thereof adapted to be scanned by an electron beam and a uni-potential imperforate secondary electron emitting structure on the opposite side of said target adapted to be scanned by another electron beam, the said emitting structure having non-uniform secondary electron emitting properties over its exposed area when said area is scanned by said other electron beam.

HARLEY A. IAMS. 

